The present invention relates to an ignition apparatus for an internal combustion engine which is particularly compact, small-sized and highly resistant to electrical noise.
FIG. 4 illustrates a typical example of a known ignition apparatus for a multi-cylinder internal combustion engine. In this Figure, a plurality of ignition coils 1 are provided one for each cylinder 9 of the engine, and each of the ignition coils 1 has a primary winding and a secondary winding. The primary winding of each ignition coil 1 is connected at one end thereof to a power supply such as a storage battery (not shown) and at the other end thereof to ground through a switch 2 in the form of a power transistor. The secondary winding of each ignition coil 1 is connected at one end thereof to ground and at the other end thereof to a signal take-out assembly 7 and to a corresponding spark plug 8 which is mounted on the head of a cylinder 9 with its electrodes present in a combustion chamber 9a defined therein. Each ignition coil 1 is mounted through fastening means 4 such as screws on a cylinder block 5 atop a corresponding spark plug 8. The power transistors 2 are controlled to be turned on and off by a computerized engine control unit (ECU) 3 which is connected to receive an output signal of a signal generator 10. The signal generator 10 generates an output signal representative of crank positions of pistons 9b received in the cylinders 9 in synchronism with the rotation of a crankshaft (not shown) connected through piston rods to the pistons 9b. The ECU 3 also receives output signals of various sensors (not shown) such as a throttle sensor, an intake pressure sensor, an engine speed sensor, an engine temperature sensor, etc. for properly controlling various aspects of engine operation including the ignition timing of the respective cylinders based on the sensor outputs. When a power transistor 2 is turned off by the ECU 3, a high ignition voltage is developed across the secondary winding of a corresponding ignition coil 1 and fed to a corresponding spark plug 8, as shown by an arrow 13, so that a spark is generated between the electrodes of the spark plug 8 to fire an air/fuel mixture in the combustion chamber 9a in a corresponding cylinder 9. The signal take-out assembly 7 includes an ion current sensing diode 6 which has an anode connected to the spark plug 8 and a cathode connected to an ion current sensing unit 11 for sensing an ion current generated in a gap between the electrodes of the spark plug 8 during or immediately after the combustion of the mixture. The ion current thus generated is fed from the spark plug 8 to the ion current sensing unit 11 through the ion current sensing diode 6, as indicated by an arrow 15. The ion current sensing unit 11 is formed separately from the switches 2 and the ignition coils 1 and housed in a metal casing. The ion current sensing unit 11 includes a signal processor 12 connected to the spark plugs 8 through a resistor 16, a capacitor 17 in the unit 11 and the respective diodes 6 in the signal take-out assemblies 7 for generating an output signal when an ion current input thereto exceeds a prescribed level, the output signal of the signal processor 12 being fed to the ECU 3, as shown by an arrow 18. The ion current sensing unit 11 further includes: a plurality of parallel circuits respectively connected at their one ends to the primary windings of the corresponding ignition coils 1 through a wire harness 14, these parallel circuits each comprising a resistor 19, a capacitor 20, a diode 21 and a diode 22 connected to each other as shown in FIG. 4; a capacitor 23 connected at one end thereof to ground and at the other end thereof to a junction 24 at which the other ends of the parallel circuits are connected together; and a resistor 25 connected at one end thereof to the junction 24 and at the other end thereof to one end of the capacitor 17 of which the other end is connected the signal processor 12. Upon deenergization of a power transistor 2, there is generated across the primary winding of a corresponding ignition coil 1 a positive voltage in the form of a pulse, as shown at (a) in FIG. 5, which is fed to the ion current sensing unit 11 where it is differentiated by the corresponding resistor 19 and the capacitor 20 to provide a differentiated voltage, as shown at (b) in FIG. 5, which is then fed through the corresponding diode 22 to the capacitor 23. The capacitor 23 provides at the junction 24 a negative voltage -Vo having a waveform, as shown at (c) in FIG. 5, which acts as a negative voltage source. The ignition coils 1, the power transistors 2, the spark plugs 8 and the ion current sensing unit 11 are separately formed from each other.
In operation, when the ECU 3 turns off one of the power transistors 2 for cylinder #1, for example, at appropriate timing, the power supply to the primary winding of the ignition coil 1 for cylinder #1 is cut off so that there is generated a high voltage across the secondary winding of the ignition coil 1 which is fed to the corresponding spark plug 8 through the corresponding diode assembly 7. As a result, the spark plug 8 generates a spark between the electrodes thereof whereby an air/fuel mixture in the combustion chamber 9a in the corresponding cylinder 9 is fired to combust. In this case, the high negative voltage thus generated across the secondary winding of the spark plug 8 is not transmitted to the ion current sensing unit 11 since the capacitor 23 acts as a negative voltage source, as referred to before. During the combustion of the mixture, there is developed an ion current in a gap between the electrodes of the spark plug 8 which is then fed through the corresponding diode 6 to the ion current sensing unit 11, as indicated by the arrow 15, which is biased to a negative voltage. The signal processor 12 processes the ion current thus fed to the ion current sensing unit 11 and generates an ion current sensing signal to the ECU 3 which determines, based on the ion current sensing signal and the crank angle signal from the signal generator 10, whether normal combustion takes place in cylinder #1.
With the above-described known ignition apparatus, the ignition coils 1, the power transistors 2, the spark plugs 8 and the ion current sensing unit 11 are all separately formed from each other and electrically connected to each other through wiring or wiring harnesses. Accordingly, in cases where the ignition apparatus is mounted in a generally narrow space such as an engine room of a motor vehicle, the entire dimensions of the above component elements become substantial and require a relatively large installation space, making it difficult to properly arrange them within the narrow engine room. In addition, the known ignition apparatus includes many discrete component elements, which results in reduced reliability in operation.
Moreover, due to limited space availability inside the vehicle engine room, the ion current sensing unit 11 has sometimes to be disposed remote from the other elements such as the ignition coils 1, the switches 2, etc., of the ignition apparatus, and it is connected to the primary windings of the ignition coils 1 and to the spark plugs 8 through the diode assemblies 7 by way of the relatively long wire harness 14 and wiring 26, which are liable to be subject to influences of electrical noise from various other electrical and electronic elements or devices disposed in the engine room. In addition, if the wire harness 14 connecting between the ion current sensing unit 11 and the ignition coils 1 is disposed in the vicinity of the wiring 26 connecting between the ion current sensing unit 11 and the diode assemblies 7, the wire harness 14, through which a high voltage passes from the primary windings of the ignition coils 1 to the ion current sensing unit 11, becomes a noise source whereas the wiring 26, through which a relatively weak ion current passes, becomes a noise recipient. As a result, the ion current in the wiring 26 tends to include electrical noise due to influences from a high voltage in the wire harness 14.